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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_optional.tutorial.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
      Factories</a>
</h3></div></div></div>
<p>
        One of the typical problems with wrappers and containers is that their interfaces
        usually provide an operation to initialize or assign the contained object
        as a copy of some other object. This not only requires the underlying type
        to be <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, but also requires
        the existence of a fully constructed object, often temporary, just to follow
        the copy from:
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span>
<span class="special">{</span>
    <span class="identifier">X</span> <span class="special">(</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">)</span> <span class="special">;</span>
<span class="special">}</span> <span class="special">;</span>

<span class="keyword">class</span> <span class="identifier">W</span>
<span class="special">{</span>
    <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>

    <span class="keyword">public</span><span class="special">:</span>

    <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
<span class="special">}</span> <span class="special">;</span>

<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Temporary object created.</span>
    <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        A solution to this problem is to support direct construction of the contained
        object right in the container's storage. In this scheme, the user only needs
        to supply the arguments to the constructor to use in the wrapped object construction.
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
<span class="special">{</span>
    <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>

    <span class="keyword">public</span><span class="special">:</span>

    <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
    <span class="identifier">W</span> <span class="special">(</span> <span class="keyword">int</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">a1</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
<span class="special">}</span> <span class="special">;</span>

<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Wrapped object constructed in-place</span>
    <span class="comment">// No temporary created.</span>
    <span class="identifier">W</span> <span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        A limitation of this method is that it doesn't scale well to wrapped objects
        with multiple constructors nor to generic code were the constructor overloads
        are unknown.
      </p>
<p>
        The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span>
        and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories
        are a family of classes which encapsulate an increasing number of arbitrary
        constructor parameters and supply a method to construct an object of a given
        type using those parameters at an address specified by the user via placement
        new.
      </p>
<p>
        For example, one member of this family looks like:
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">A1</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span>
<span class="special">{</span>
    <span class="identifier">A0</span> <span class="identifier">m_a0</span> <span class="special">;</span> <span class="identifier">A1</span> <span class="identifier">m_a1</span> <span class="special">;</span>

    <span class="keyword">public</span><span class="special">:</span>

    <span class="identifier">TypedInPlaceFactory2</span><span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">a1</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">m_a0</span><span class="special">(</span><span class="identifier">a0</span><span class="special">),</span> <span class="identifier">m_a1</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>

    <span class="keyword">void</span> <span class="identifier">construct</span> <span class="special">(</span> <span class="keyword">void</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">)</span> <span class="special">{</span> <span class="keyword">new</span> <span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">m_a0</span><span class="special">,</span><span class="identifier">m_a1</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
 <span class="special">}</span> <span class="special">;</span>
</pre>
<p>
        A wrapper class aware of this can use it as:
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
<span class="special">{</span>
    <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>

    <span class="keyword">public</span><span class="special">:</span>

    <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
    <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="identifier">construct</span><span class="special">(&amp;</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
<span class="special">}</span> <span class="special">;</span>

<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.</span>
    <span class="comment">// No temporary created.</span>
    <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">))</span> <span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        The factories are divided in two groups:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            <span class="underline">TypedInPlaceFactories</span>: those which
            take the target type as a primary template parameter.
          </li>
<li class="listitem">
            <span class="underline">InPlaceFactories</span>: those with a
            template <code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code>
            member function taking the target type.
          </li>
</ul></div>
<p>
        Within each group, all the family members differ only in the number of parameters
        allowed.
      </p>
<p>
        This library provides an overloaded set of helper template functions to construct
        these factories without requiring unnecessary template parameters:
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">&gt;</span>
<span class="identifier">InPlaceFactoryN</span> <span class="special">&lt;</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">&gt;</span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">&gt;</span>
<span class="identifier">TypedInPlaceFactoryN</span> <span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">&gt;</span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
</pre>
<p>
        In-place factories can be used generically by the wrapper and user as follows:
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
<span class="special">{</span>
    <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>

    <span class="keyword">public</span><span class="special">:</span>

    <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>

    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">&gt;</span>
    <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="keyword">template</span> <span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span><span class="identifier">construct</span><span class="special">(&amp;</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>

<span class="special">}</span> <span class="special">;</span>

<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
<span class="special">{</span>
    <span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.</span>
    <span class="comment">// No temporary created.</span>
    <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">in_place</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        The factories are implemented in the headers: <a href="../../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a>
        and <a href="../../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a>
      </p>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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